Conventionally, a seat slide locking apparatus is known, such as the apparatus disclosed in Japanese Examined Utility Model Application Publication No. S47-39764. The apparatus disclosed in Japanese Examined Utility Model Application Publication No. S47-39764 comprises a plurality of locking piece accommodating sections (recesses) which are provided in a lower rail that is fixed to a floor and which are arranged in a front/rear direction following a lengthwise direction of the lower rail, and one locking piece which is provided in an upper rail installed in a fixed fashion to a seat and which has a locking hook that interlocks with and is received in the locking piece accommodating sections. The locking piece is locked by being inserted into a suitable one of the locking piece accommodating sections, in such a manner that the seat can be fixed in the front/rear position.